Researchers from this study suspect that up to one fifth of breast
cancer may spontaneously regress if left alone. The discovery was made
by accident while they were researching the effects of regular mammogram
screening. They compared cumulative breast cancer incidence in
age-matched cohorts of women residing in 4 Norwegian counties before and
after the initiation of biennial mammography, which started in 1996.

The screened group included all women who were invited for all 3
rounds of screening during the period 1996 through 2001 (age range in
1996, 50-64 years). The control group included all women who would have
been invited for screening if there had been a screening program during
the period 1992 through 1997 (age range in 1992, 50-64 years). All
participants were invited to undergo a one-time screening to assess
breast cancer prevalence at the end of their observation period.
Screening attendance was similar in both groups (screened 78.3%,
controls 79.5%).

As expected the cumulative incidence of invasive breast cancer was
significantly higher in the screened group than in the controls. But
when the controls had their one-time mammogram six years later, in many
cases the cancer that had been detected at the beginning of the study
had gone. Overall the cancer rate was 22% higher in the women who were
regularly screened.

Because the cumulative incidence among controls never reached that
of the screened group, it appears that some breast cancers detected by
repeated mammographic screening would not persist to be detectable by a
single mammogram at the end of 6 years. The most likely possible
explanation for this, according to the researchers, is that the natural
course of some screen detected invasive breast cancers is to
spontaneously regress.

As the researchers point out, medical ethics does not allow cancer
to follow its natural path without intervention and so there is very
little evidence of this phenomenon. Nonetheless there are many
individual cases of cancers that naturally regress, and this is
recognised as a possible outcome of cases of metastatic melanoma and
metastatic renal cell carcinoma in particular.